The present invention relates to a switching power supply which is provided with a standby mode.
Conventionally, there are widely used switching power supplies for switching a direct current, which is obtained by rectifying and smoothing a commercial alternating current, with a high frequency of approximately 100 kHz to convert the direct current to a voltage of a desired value using a transformer with a high efficiency ratio.
As methods for controlling an output voltage of these switching power supplies, there are employed the PWM (Pulse Width Modulation) method which controls the duty ratio of a switching pulse corresponding to the variation of an output voltage, the frequency controlling method and the phase controlling method which controls frequency and phase of a switching pulse, respectively, such as resonating, etc.
Conventionally, in a switching power supply used in such as an AC adapter, a converter continues to perform switching operation in series. Thus, power for driving a switching element of the converter and power consumed at a controlling circuit is large, and the ratio of the power against output power is large. So, not only the conversion efficiency is lowered, but also restraining power consumption at the standby state is difficult.
To cope with this inconvenience, there is practically used a switching power supply provided with the standby mode which can reduce power for driving a switching element of the converter and power consumed at a controlling circuit and lessen power consumption at the standby state by observing the outputting state to detect the standby state and changing the operation mode of a switching element of a converter from ordinary serious oscillation state to intermittent oscillation state, as shown in FIG. 1.
In observing the outputting state, for example, as shown in FIG. 2, a detection resistor Rs is connected in series to an output line, and the value of a voltage xcex94Vs between ends of the detection resistor Rs is detected for judgement.
However, in a conventional switching power supply, when coming into the standby state, an output voltage is caused to be 0 or an extremely small value. Thus, in order to keep the value of the voltage xcex94Vs between ends of the detection resistor Rs higher than a value of a certain extent at the standby state, the value of the detection resistor Rs is required to be enlarged. In this case, since loss of power caused by the detection resistor Rs becomes large in the ordinary operation state, a conventional switching power supply has a switching element Q1 connected in parallel to the detection resistor Rs to shunt the output line in the ordinary operation state to reduce loss of power. On the other hand, there is raised a problem that increase of the number of parts and components and new loss of power caused by such countermeasure prevent a switching power supply from being reduced in size and price.
Accordingly, the present invention has an object to overcome the above-mentioned drawbacks of the prior art by providing a switching power supply which does not have a detection resistor causing loss of power, whose operation mode can be changed to the standby mode, enabling the reduction in size and price of the apparatus itself.
The above object can be attained by providing a switching power supply, including:
switching means for switching a direct input;
switching control means for controlling the switching operation of the switching means;
a converter transformer whose primary winding is supplied with a switching output from the switching means;
main rectifying and smoothing means and secondary rectifying and smoothing means for rectifying and smoothing an output obtained at a secondary winding of the converter transformer; and
error detection means, which is driven by a secondary rectified and smoothed output from the secondary rectifying and smoothing means, for detecting a main rectified and smoothed output from the main rectifying and smoothing means to be supplied to loads so as to control the switching operation of the switching means by the switching control means so that the main rectified and smoothed output is caused to be a predetermined state, and for comparing the voltage of the secondary rectified and smoothed output with the voltage of the main rectified and smoothed output so as to change the operation state of the switching control means to the standby mode in case the voltage of the secondary rectified and smoothed output declines to come below the voltage of the main rectified and smoothed output by a predetermined value.